In order to make a distributed feedback laser that use of oscillates in a longitudinal fundamental mode, a .lambda./4-shifted region in a central portion of a diffraction grating in the laser has been generally known.
FIGS. 2(a) to 2(e) show the steps of a prior art method of producing a phase shift type diffraction grating. In these figures, reference numeral 1 designates a semiconductor substrate. A negative resist 12 is deposited on the substrate 1. An intermediate layer 13 is deposited on the negative resist 12 followed by a first positive resist 14. Intermediate layer 13 prevents mixing of the resists 12 and 14. OBC (Ohka Back Coating), which is of the same material as the negative resist, but which contains no photosensitive radicals, is used as the intermediate layer 13. Reference numeral 15 designates a second positive resist. An oxygen isolating film 16 is deposited on the second positive resist 15 for preventing the resist from chemically reacting with oxygen during exposure. PVA (Polyvinyl Alcohol), which is the same material as the positive resist, but which contains no photosensitive radicals, is used as the oxygen isolating film 16.
The production process will be described.
A negative resist 12, an intermediate layer 13, and a first positive resist 14 are deposited on a semiconductor substrate 1, as shown in FIG. 2 (a). A portion of the first positive resist 14 is then removed using ordinary photolithographic techniques, as shown in FIG. 2 (b). Utilizing a pattern of the first positive resist 14 as a mask, a portion of the intermediate layer 13 and a portion of the negative resist 12 are etched away by using a solution of nitric acid as an etchant, as shown in FIG. 2(c). The first positive resist 14 is removed and then a second positive resist 15 and an oxygen isolating film 16 are deposited, as shown in FIG. 2(d). In this state, exposure of layer 16 to interference fringes of light is made as shown in FIG. 2 (e), followed by development and rinsing, to form a pattern shown in FIG. 2(f) comprising the negative resist 12 and the second positive resist 15. The substrate 1 is etched utilizing this pattern as a mask and then the negative resist 12 and the second positive resist 15 are removed. Consequently, a phase shift type diffraction grating shown in FIG. 2 (g) is obtained.
The prior art method of producing a .lambda./4-shifted diffraction grating comprising the foregoing steps has disadvantages in that the steps are complicated and take much time.